Atom-atom interaction model and internal vibrational exciton splittings in organic charge-transfer complexes: Factor group splittings of naphthalene vibrations in several crystalline complexes

Factor group splittings of naphthalene vibrations are experimentally investigated for naphthalene: 2SbCl₃ (C₁₀H₈ - 2SbCl₃), naphthalene: octafluoronaphthalene (C₁₀H₈ : C₁₀F₈), naphthalene: TCNB, and naphthalene: TNB crystalline complexes by Raman spectroscopy and using the isotopically mixed crystal technique. It is found that only 386 cm^?1 mode of C₁₀H₈ shows factor group splittings in the first two complexes. The splitting increases from 5 cm^?1 in pure C₁₀H₈ crystal to 5.5cm^?1 in C₁₀H₈ : 2SbCl₃ but decreases to 1cm^?1 in the C₁₀H₈ : C₁₀F₈ complex. Also as SbCl₃ is successively replaced by SbBr₃ in the complex C₁₀H₈ : 2SbCl₃, the factor group splitting of 386 cm^?1 C₁₀H₈ mode decreases and the mean of the factor group frequencies goes through a minimum near 0.5 mole fraction of SbBr₃. A theoretical calculation using atom-atom potential model and considering only naphthalene-naphthalene interactions predicts that the factor group splitting on 386 cm^?1 band should increase from pure C₁₀H₈ crystal to the C₁₀H₈ :2SbCl₃ crystalline complex and decrease in C₁₀H₈ : C₁₀F₈. However, the calculation also predicts a similar trend for 943 cm^?1 band of naphthalene which shows a factor group splitting of 5 cm^?1 in pure C₁₀H₈ but none in the C₁₀H₈ : 2SbCl₃ complex. Furthermore, the atom-atom interaction model does not explain the effect of SbBr₃ substitution on the factor group splitting. The importance of electrostatic multipole interactions in explaining the behavior of factor group splitting is discussed.     

Ultrafast intramolecular exciton splitting dynamics in isolated low-band-gap polymers and their implications in photovoltaic materials design

Record-setting organic photovoltaic cells with PTB polymers have recently achieved ~8% power conversion efficiencies (PCE). A subset of these polymers, the PTBF series, has a common conjugated backbone with alternating thieno[3,4-b]thiophene and benzodithiophene moieties but differs by the number and position of pendant fluorine atoms attached to the backbone. These electron-withdrawing pendant fluorine atoms fine tune the energetics of the polymers and result in device PCE variations of 2-8%. Using near-IR, ultrafast optical transient absorption (TA) spectroscopy combined with steady-state electrochemical methods we were able to obtain TA signatures not only for the exciton and charge-separated states but also for an intramolecular ("pseudo") charge-transfer state in isolated PTBF polymers in solution, in the absence of the acceptor phenyl-C(61)-butyric acid methyl ester (PCBM) molecules. This led to the discovery of branched pathways for intramolecular, ultrafast exciton splitting to populate (a) the charge-separated states or (b) the intramolecular charge-transfer states on the subpicosecond time scale. Depending on the number and position of the fluorine pendant atoms, the charge-separation/transfer kinetics and their branching ratios vary according to the trend for the electron density distribution in favor of the local charge-separation direction. More importantly, a linear correlation is found between the branching ratio of intramolecular charge transfer and the charge separation of hole-electron pairs in isolated polymers versus the device fill factor and PCE. The origin of this correlation and its implications in materials design and device performance are discussed.     

Highly luminescent InP/GaP/ZnS nanocrystals and their application to white light-emitting diodes

Highly stable and luminescent InP/GaP/ZnS QDs with a maximum quantum yield of 85% were synthesized by in situ method. The GaP shell rendered passivation of the surface and removed the traps. TCSPC data showed an evidence for the GaP shell. InP/GaP/ZnS QDs show better stability than InP/ZnS. We studied the optical properties of white QD-LEDs corresponding to various QD concentrations. Among various concentrations, the white QD-LEDs with 0.5 mL of QDs exhibited a luminous efficiency of 54.71 lm/W, Ra of 80.56, and CCT of 7864 K.     

Conditions for the formation of superconductive polymers and possibilities for their realization

General principles of the creation of superconductivity at the polymers are considered. It is shown that high temperature superconductivity may be produced only at really one-dimensional conducting materials. It is also shown that conjugated polymers cannot have onedimensional conductivity. The new class of nonconjugated polar elastomers is proposed as a perspective one for the creation of high temperature superconductive structures–superpolarons–which appear as a result of internal selfionization of the polymer matrix and a consequent selforganization of free polarons. The practical methods of the formation of superpolarons are shown for the films of oxidized atactice polypropylene. Evidences are given which show the existence of superconductive structures in oxidized polypropylene at room temperature.     

Helix formation in linear achiral dendronized polymers: a computer simulation study

We present a molecular simulation study of the structure of linear dendronized polymers. We use excluded volume interactions in the context of a generic coarse grained molecular model whose geometrical parameters are tuned to represent a poly(paraphenylene) backbone with benzyl ether, Frechet-type dendrons. We apply Monte Carlo sampling in order to investigate the formation of packing-induced chiral structures along the polymer backbone of these chemically achiral systems. We find that helical structures can be formed, usually with defects consisting of domains with reversed helical handedness. Clear signs of helical arrangements of the dendrons begin to appear for dendritic generation g=4, while for g=5 these arrangements dominate and perfect helices can be observed as equilibrium structures obtained from certain types of starting configurations.     

Rapid photoresponses in the retina and their relevance to vision research

Abstract— An intense short flash stimulus gives rise to transient electrical responses of about a millisecond duration from vertebrate and some invertebrate retinas. Some aspects of these rapid retinal responses are reviewed. It is concluded that they probably represent electrical expressions of photochemical events or events closely associated with photochemistry of visual pigment, that they do not represent the excitation of the photoreceptors per se, and that still unidentified, silent processes intervene between them and the excitation of the photo-receptor. The fact that photochemical events produce movements of charges, which result in these responses, is of interest in itself. Moreover, these responses present us with a relatively simple and effective technique for probing the role of visual pigment in visual excitation in the intact retina.     

TFC polyamide membranes modified by grafting of hydrophilic polymers: an FT-IR/AFM/TEM study

Surface modification using grafting of a hydrophilic polymer onto the membrane surface is a possible route to improving the fouling properties of polyamide thin-film composite membranes. The structure of nanofiltration (NF) and reverse osmosis (RO) membranes modified using graft polymerization of acrylic (AA) monomers was visualized and analyzed using attenuated total reflection–Fourier transform infrared spectroscopy, atomic force microscopy and transmission electron microscopy. The results show that a layer of AA polymer is indeed formed on the polyamide surface, which could be accompanied by a change of the surface morphology. It was observed that for the NF membranes studied polymerization could also take place inside the pores of the support as a result of penetration of the monomer through the active layer, particularly for high degrees of grafting. It suggests that the modification procedures should be optimized so that the latter effect is minimized.     

Morphology design of microporous organic polymers and their potential applications: an overview

Microporous organic polymers (MOPs) have attracted considerable research interest because of their well-defined porosity, high surface area, lightweight nature, and tunable surface chemistry. The morphology of MOPs are demonstrated to play a significant role in various applications although limited examples manifesting the importance of the MOP morphology in numerous applications have been reported. This review summarizes the recent progress in the design of MOPs using different techniques, including hard and soft template and direct synthesis methods. In addition, their applications, which possibly attribute to their shape, are discussed. Furthermore, the advantages and disadvantages of different methods are discussed, as well as their development and future challenges.     

Characterization of acid-base properties of polymers and other materials: relevance to adhesion science and technology

This paper reviews the background to the theory of Lewis acid-base (AB) interactions in adhesion, adsorption, wetting and mixing of polymers and other materials (pigments, fillers, fibres, etc.). These specific materials interactions require the revision of old concepts («polar» interactions) and the development of new analytical techniques and methodologies. Four of the most currently used techniques to characterize AB interactions are described: contact angle measurements, inverse gas chromatography. X-ray photoelectron spectroscopy, and atomic force microscopy.     

Star-branched polymers in an adsorbing slit: a Monte Carlo study

A coarse-grained model of star-branched polymer chains confined in a slit was studied. The slit was formed by two parallel impenetrable surfaces, which were attractive for polymer beads. The polymer chains were flexible homopolymers built of identical united atoms whose positions in space were restricted to the vertices of a simple cubic lattice. The chains were regular star polymers consisted of f = 3 branches of equal length. The chains were modeled in good solvent conditions and, thus, there were no long-range specific interactions between the polymer beads-only the excluded volume was present. Monte Carlo simulations were carried out using the algorithm based on a chain's local changes of conformation. The influence of the chain length, the distances between the confining surfaces, and the strength of the adsorption on the properties of the star-branched polymers was studied. It was shown that the universal behavior found previously for the dimension of chains was not valid for some dynamic properties. The strongly adsorbed chains can change their position so that they swap between both surfaces with frequency depending on the size of the slit and on the temperature only.     

Relation of gas-transport parameters of amorphous glassy polymers to their free volume: Positron annihilation study

Correlations of transport parameters (diffusion coefficients D and permeabilities P of gases) and thermodynamic parameters (solubility coefficients S and parameters C′ _H of sorption isotherms) with the sizes of free-volume elements, v _h, as estimated via positron annihilation lifetime spectroscopy are analyzed for the first time on the basis of the data array obtained for glassy polymers. Correlations of logD and logP with 1/v _h that agree with the free-volume model under the condition of a weak change in the concentration of free-volume elements in different polymers are ascertained. Certain deviations from linear correlations with 1/v _h for polymers with high free volumes are interpreted as evidence that the connectivity (openness) of pores increases with the sizes of free-volume elements. For solubility coefficients and Langmuir parameters of sorption capacity C′ _H , good linear correlations with the value of v _h are demonstrated.     

Singlet-triplet energy gaps in highly stabilized nitrenium ions: experimental and theoretical study of 1,3-dimethylbenzotriazolium Ion

The first experimental determination of a singlet-triplet energy gap (DeltaE(st)) for an organic nitrenium ion was made for 1, 3-dimethylbenzotriazolium ion 1. Laser flash photolysis was used to determine DeltaE(st) for this persistent nitrenium ion and a value of -66 +/- 3 kcal mol(-)(1) was obtained. DFT calculations show excellent agreement with the experiment.     

Structural aspects in polymers: interconnections between conformational parameters of the polymers with their physical properties

This article focuses on the influence of the geometric structure of the polymer chain on the physical properties of polymers. It is shown that for polyheteroarylenes the Kuhn segment calculated based on the geometric structure of the polymer chain has a good correlation with the physical properties of those polymers. For aliphatic polymers, a volume factor is suggested that equals the ratio of the Van der Waals volume of pendant substitute to the volume of the whole polymer chain. It is shown that the properties of aliphatic polymers that depend on free volume are well described by this value.     

EOM/Propagator ionization potentials: Effect of an extended operator manifold on the predicted valence-shell ionization spectra of ethane,ethylene, and acetylene

The orthogonalized direct diagonalization (ODD ) method is used to predict the valence-shell ionization spectra for the series of hydrocarbons ethane (C₂H₆), ethylene (C₂H₄), and acetylene (C₂H₂) within an EOM /propagator formalism. Both (a) third-order and (b) higher-order (with h₅ terms in the excitation operator manifold) calculations are presented in order to illustrate the effect of the extended operator manifold on the predicted valence-shell spectra. It is shown that the major effect of the h₅ manifold is concentrated in its “occupied” part. Along with a general shifting of the main lines for outer valence ionizations to lower values (together with a slight reduction in pole strength), it is found that the effect of the h₅ manifold in the inner valence region is dependent on the system being studied.     

Ab initio study of ladder-type polymers: Polythiophene and polypyrrole

An ab initio study of four polymers, polythiophene, polypyrrole, ladder-type polythiophene, and ladder-type polypyrrole is presented. Upon an analysis of the variation of the band gap when comparing the non-ladder and the ladder-type polymers, a discrepancy is found between the thiophene and the pyrrole polymer families. For polythiophene, the ladder-type polymer has a larger band gap than the non-ladder polymer whereas the opposite is found for the pyrrole polymers. This difference is explained in terms of the competition between the bond length alternation and the effect of the charge density distribution in the carbon backbone.